1. Field of the Invention
The present invention relates to a transmitting/receiving system between private branch exchanges (PBX's) which includes a plurality of analog junction lines linked between one PBX provided with a plurality of integrated service digital network (ISDN) terminal equipment and another PBX provided with a plurality of ISDN terminal equipment and a plurality of analog tie trunks connected to the network constituting the one PBX and the network constituting the other PBX and connected to ends of the corresponding analog junction lines.
An ISDN is a digital integrated communications network with a wide range of services which uses digital lines. It is widely spreading in use. At the present, however, there are a few users which would like to use services of the ISDN other than its data communications service, i.e., just the telephone services of the ISDN. It is possible for users to make use of the abundant telephone services of an ISDN by using as is existing, cheap analog lines instead of introducing the higher cost digital lines.
2. Description of the Related Art
The spread of ISDN in recent years has been accompanied by the replacement of subscribers' terminal equipment with ISDN compatible terminal equipment. An increasing number of users would like to use just the telephone services among the various services available with an ISDN for the time being. In this case, the user cannot receive such ISDN telephone services just by replacing his digital terminal equipment with ISDN terminal equipment. That is, to receive such services, he must install some additional means.
Such additional means exist in concept, but none has yet actually been seen. If such an additional means were to be extremely high in price, there would be the problem that all significance would be lost in the users making use of existing analog line equipment.